This invention relates to methods of manufacturing an electrical transducer device (particularly, but not exclusively, an infrared detector array) comprising an electrically active film of polymer material. It relates particularly, but not exclusively, to the provision of a fine-geometry pattern of thin electrodes at one face of a copolymer film with pyroelectric and/or ferroelectric properties for a linear array or 2-dimensional array of infrared detector elements, for example for a comparatively inexpensive thermal-image sensing device or a camera. The invention also relates to electrical transducer devices manufactured by such methods, and to such devices comprising a polymer film with pyroelectric and/or ferroelectric and/or piezoelectric properties and having electrodes supported by the film.
In the manufacture of thermal detectors and other transducer devices there is a current trend towards adopting electrically active films of polymer materials, for example polyvinylidene fluoride (PVDF) or vinylidene fluoride trifluoroethylene to form the pyroelectric and/or ferroelectric active dielectric layer of the infrared detector elements (instead of using the formerly more conventional ceramic materials such as, for example, lanthanum-doped lead zirconate titanate, PLZT). Many publications have occurred on how devices can be constructed using these polymer materials. By way of example, reference is made to published European patent application EP-A-0 269 161 which corresponds to U.S. Pat. No. 4,806,763 and to U.S. Pat. No. 4,532,424 and 4,250,384, the whole contents of which are hereby incorporated herein as reference material.
These known polymer-film devices comprise a group of first electrodes at a first face of the film and at least one second electrode at the second face of the film opposite the first electrodes so as to define with the intermediate areas of the film a group of the infrared detector elements. Due to the pyroelectric properties of the film, electric charges are generated at first and second opposite faces of the film in operation of the device, and the first and second electrodes form electrical connections to the film for detecting the generated charges. It is customary to apply both the first and second electrodes to a preformed film by coating the film with electrode material, or possibly in some cases by laminating the preformed film with electrodes which may be carried on or in a support. A pattern of the first electrodes is normally defined by localized deposition of the electrode material through openings in a stencil mask held in contact with the film or with the support on which the electrodes are being deposited. A support (such as a glass slide) is often employed in experimental test structures and may sometimes be used in manufactured devices (for example a glass or plastic substrate may form a lens pattern in the device structure). Usually, however, in commercial, high performance infrared detectors of pyroelectric and/or ferroelectric polymer film material, at least most of the area of the film where the detector elements are present is free of contact with the mount arrangement used to support the film in the device, so as to reduce thermal conduction to and from the detector elements. In the specific arrangement illustrated by way of example in EP-A-0 269 161, the electroded film is supported at its periphery on an insulating annulus. In the specific embodiment of U.S. Pat. No. 4,532,424, bumped contacts form the mounting arrangement to space the electroded film above a semiconductor signal-processing circuit. In specific embodiments of U.S. Pat. No. 4,250,384, electroded etched dips are present in a semiconductor substrate to provide thermal barriers in the mounting arrangement of the electroded film.
The present inventor has found that the use of a stencil mask to define the group of first electrodes is not always satisfactory, especially for the definition of fine-geometry patterns and so has considered using a photolithographic process for this purpose. However, the inventor has experienced some difficulties in carrying out photolithography directly on several such polymer films, since the polymer film material may dissolve (very readily in some cases) in organic solvents which are normally used either to develop the photolithographic polymer mask pattern or the remove subsequently this polymer mask pattern from the film. Degradation of the film properties (e.g. pyroelectric and ferroelectric properties) may also occur by contamination from the photoresist mask. The use of lamination techniques to apply electrode patterns to polymer films has also not proved satisfactory in terms of reliability and reproducibility. Furthermore, studies leading to the present invention indicate that improved pyroelectric and/or ferroelectric infrared detector element characteristics may be obtained by having a thin polymer film with very thin first electrodes (of even smaller thickness than the small thickness of the film) embedded in one face of the film which is supported free of contact with the mount arrangement of the device, at least over most of the area where the group of infrared detector elements are present.
Most of the electrically active polymer materials used for pyroelectric and/or ferroelectric infrared detectors are also piezoelectric. Similar problems to those described above can arise in providing fine-geometry patterns of electrodes for other electrical transducer devices (i.e. not only in infrared detectors), which have piezoelectric polymer films between first and second electrodes.